Recently, due to the exhaustion of fossil energy and environmental pollution, the interest in electric products, which may operate with electrical energy instead of fossil energy, is growing more and more. Accordingly, with the increasing development and demand of mobile devices, electric vehicles, hybrid vehicles, energy storage devices, uninterrupted power supplies, or the like, the demand of secondary batteries as energy sources are rapidly increasing as well as its various forms thereof. Therefore, secondary batteries are being studied very actively to cope with such diverse demands.
Generally, a secondary battery includes a nickel cadmium battery, a nickel metal hydride battery, a lithium ion battery, a lithium ion polymer battery, and the like. This secondary battery is largely classified into a lithium-based battery and a nickel metal hydride-based battery. The lithium-based battery is primarily used in a wide range of application including small-scale products, for example, a digital camera, a P-DVD, an MP3P, a mobile phone, a PDA, a Portable Game Device, a Power Tool, an E-bike, and the like, and the nickel metal hydride-based battery is used in a variety of application including large-scale products requiring high output such as an electric vehicle or a hybrid electric vehicle.
Meanwhile, to drive an electric vehicle or a hybrid electric vehicle, an electric motor requiring high output should be operated. Also, in a case of an energy storage device used to supply power to a building or a predetermined area, a sufficient amount of energy to meet the energy demand must be supplied. Accordingly, energy of a desired output or amount is provided using a battery pack in which a plurality of unit secondary battery cells, hereinafter referred to as unit cells, are connected in series or in parallel, to provide high output or high capacity energy.
However, in a case of a battery pack including a plurality of unit cells connected to one another, a charge capacity difference occurs between the respective unit cells as charging and discharging is performed repeatedly. Under the condition of charge capacity imbalance, a particular unit cell having a low charge capacity is over-discharged if the battery pack is continuously discharged, which hinders a stable operation of the battery pack. In contrast, under the condition of charge capacity imbalance, a particular unit cell having a high charge capacity is over-charged if the battery pack is continuously charged, which reduces safety of the battery pack. The charge capacity imbalance puts a certain unit cell into an over-charge state or over-discharge state and thereby hinders stable power supply to a load, for example, an electric motor or an electrical grid. To solve this problem, a charge capacity balancing operation is needed to continuously monitor and balance the charge capacity of unit cells to a predetermined level.
This charge capacity balancing operation is performed in the way of discharging a unit cell having a higher charge capacity than a reference charge capacity through a buck circuit until it reaches the reference charge capacity, or charging a unit cell having a lower charge capacity than the reference charge capacity through a boost circuit until it reaches the reference charge capacity.
However, it takes some time to charge or discharge a secondary battery due to the characteristics of the secondary battery, and as a charge or discharge amount increases and the number of unit cells requiring a balancing operation increases, a period of time taken to complete the balancing operation increases. Further, since use of a battery pack is restricted while a balancing operation of unit cells is in progress, there is a need to control a time to start the balancing operation.